The liquid crystal display device is classified based on the operation mode of liquid crystals, for example, into PC (phase change), TN (twisted nematic), STN (super twisted nematic), BTN (bistable twisted nematic), ECB (electrically controlled birefringence), OCB (optically compensated bend), IPS (in-plane switching), and VA (vertical alignment). The device is classified based on the driving system as PM (passive matrix) and AM (active matrix). PM (passive matrix) is classified into static, multiplex, etc. and AM is classified into TFT (thin film transistor), MIM (metal insulator metal), etc.
The liquid crystal display devices contain liquid crystal compositions. In the following description, the liquid crystal display device is sometimes referred to merely as a device. The liquid crystal composition is sometimes referred merely to as a composition. The liquid crystalline compound is sometimes referred to merely as a compound. In order to improve the characteristics of the device, it is preferred that the composition has appropriate physical properties. General physical properties necessary for compound as an ingredient of the composition are as described below.    (1) Chemical stability and physical stability    (2) High clearing point    (3) Low lower limit temperature of a liquid crystal phase    (4) Low viscosity    (5) Appropriate optical anisotropy    (6) Appropriate dielectric anisotropy    (7) High specific resistivity
The clearing point is a transition temperature from liquid crystal phase to isotropic phase. The liquid crystal phase means nematic phase, smectic phase, etc. Compounds having high dielectric anisotropy often have high viscosity.
The composition is prepared by mixing various compounds. Accordingly, it is preferred that the compounds are well mixed with other compounds. Since the device is sometimes used at a temperature below a freezing point, compounds having favorable compatibility at a low temperature are preferred. Compounds having high clearing point or low lower limit temperature of a liquid crystal phase contribute to a wide temperature range of the nematic phase in the composition. A preferred composition has an optical anisotropy suitable to the low viscosity and the mode of the device. A high dielectric anisotropy of the compound contributes to a low threshold voltage of the composition. Such a composition can provide a device having such characteristics as usable wide temperature range, short response time, large contrast ratio, low driving voltage, small power consumption, and large voltage holding ratio.
As a compound showing a negative dielectric anisotropy, a compound (i) having 2,3-difluoro-1,4-phenylene in a partial structure has been known generally (refer to Non-Patent Document 2). As a compound for improving the dielectric anisotropy, a compound (ii) having trifluoromethyl laterally bonded, and a compound (iii) and a compound (iv) in which difluoromethyl is bonded for making the negative dielectric anisotropy to a more larger value have been reported (refer to Patent document 1, Non-Patent Document 1, Non-Patent Document 2, and Patent Document 2). However, while such compounds show negative dielectric anisotropy of relatively large values, balance of the physical properties as the liquid crystal material is not preferred such that the clearing point is low or they show high viscosity. Further preferred liquid crystalline compounds, liquid crystal compositions, and liquid crystal display devices are demanded.
    [Patent Document 1] JP-A No. 8-040953    [Patent Document 2] Pamphlet of WO2000/39063    [Non-Patent Document 1] Synlett. 1999, No. 4, 389-396    [Non-Patent Document 2] Angew. Chem. Int. Ed. 2000, 39, 4216-4235